Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, thermal barrier coatings must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. Thermal barrier coating systems capable of satisfying the above requirements have generally required a metallic bond coat deposited on the component surface, followed by an adherent ceramic layer that serves to thermally insulate the component. Metal oxides, such as zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure. Bond coats are typically formed of an oxidation-resistant aluminum-based intermetallic such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant aluminum-containing alloy such as MCrAlY (where M is iron, cobalt and/or nickel).
As known in the art, MCrAlY bond coats are typically formed by plasma spraying, electron beam physical vapor deposition (EBPVD) and sputtering, and have a typical grain size of about 10 to 20 micrometers. Diffusion aluminide bond coats are generally formed by a diffusion process such as pack cementation, vapor phase (gas phase) aluminiding (VPA), or chemical vapor deposition (CVD). Diffusion processes react the surface of a component with an aluminum-containing composition to form two distinct zones, the outermost of which is an additive layer that contains the environmentally-resistant intermetallic phase NiAl. Beneath the additive layer is a diffusion zone (DZ) comprised of NiAl and secondary phases such as Ni.sub.3 Al (.gamma.'), solid-solutioned nickel (.gamma.), carbides and sigma phases. A typical grain size for the outermost additive layer is about 20 to 50 micrometers.
The aluminum content of the above-noted bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the ceramic layer to the bond coat. Though bond coat materials are particularly alloyed to be oxidation-resistant, the oxidation that occurs over time at elevated temperatures gradually depletes aluminum from the bond coat. Eventually, the level of aluminum within the bond coat is sufficiently depleted to prevent further slow growth of the protective oxide, and to allow for the more rapid growth of nonprotective oxides. At such time, spallation may occur at the interface between the bond coat and the aluminum oxide layer or the interface between the oxide layer and the ceramic layer.
In addition to depletion of aluminum, the ability of the bond coat to form the desired aluminum oxide layer can be hampered by the interdiffusion of elements between the superalloy and bond coat, such as during formation of a diffusion aluminide coating and during high temperature exposure. In particular, elements such as nickel, cobalt, chromium, titanium, tantalum, tungsten and molybdenum can increase the growth rate of aluminum oxide and form voluminous, nonadherent oxides or oxide scales that may be deleterious to the adhesion of the ceramic layer.
From the above, it is apparent that the service life of a thermal barrier coating is dependent on the bond coat used to anchor the thermal insulating ceramic layer. Once spallation of the ceramic layer has occurred, the component must be scrapped or refurbished at considerable cost by removing the remaining ceramic layer and bond coat, including any diffusion zone between the bond coat and substrate. Because removal of the diffusion zone effectively removes a portion of the substrate surface, a limited number of repairs can be performed before the component must be scrapped. In view of the considerable cost to repair and replace thermal barrier coating systems, there has been a considerable and continuous effort to increase their service life by improving the spallation resistance of the thermal insulating layer. However, such efforts have been complicated by the demand for higher service temperatures for gas turbine engines.